Derivative Instruments (Gains And Losses From Derivative Instruments Not Designated As Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Regulatory Assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	$ 9.5	$ (39.7)
Regulatory Assets [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	2.7	(22.2)
Regulatory Assets [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	6.8	(17.5)
Regulatory Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	16.4	1.4
Regulatory Liabilities [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	8.8	0
Regulatory Liabilities [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	$ 7.6	$ 1.4